Eraser

ABSTRACT

An eraser is constituted by an elastic material containing a rubber component or a resin component, and a skeleton structure for reinforcing the elastic material, and the skeleton structure is made from a porous structural material such as from an organic polymer that is broken when rubbed. Void portions in the porous structural material of the skeleton structure contain the elastic material of the eraser composition. Here, the skeleton portions of the skeleton structure may have an average thickness of 1 to 100 μm, and the void portions may have an average pore size of 10 μm to 3 mm. The eraser may have a surface hardness of 50 to 80, and also a sticking strength of 1.5 to 20 (kgf).

BACKGROUND OF THE INVENTION

The present invention relates to an eraser, and more particularlyconcerns an eraser which can be used with a light touch is lesssusceptible to cracking, is superior in its scrap-collecting property,and has an excellent erasing property.

An eraser (rubber eraser) for erasing handwriting written with a writingtool such as a pencil is composed of an elastic material of an erasercomposition comprising rubber components or resin components such asrubbers, plastics and thermoplastic elastomers, and additive agents suchas fillers and polishing materials.

In general, with respect to the performance required for an eraser, theerasing property and touch in use are listed; however, in recent years,the form of eraser scraps and its scrap-collecting property (combiningproperty for eraser scraps) have become important factors. Theseproperties mainly are determined by the hardness of the elastic materialof an eraser composition constituting an eraser main body. In general,the softer the elastic material, the better the erasing property and thebetter the scrap-collecting property; in contrast, as the elasticmaterial becomes softer, the touch in use becomes heavier, and it issusceptible to cracking due to repeated use and use with a highpressure. The hardness of the eraser main body can be controlled byadjusting factors such as the type, viscosity and molecular weight ofrubber components and resin components, the type and compounded ratio ofplasticizers and softeners and the type and compounded ratio of additiveagents, such as polishing agents, etc., as well as adjustingmanufacturing conditions. Thus, taking the above-mentioned trends intoconsideration, the manufacturers determine the hardness of an eraser byadjusting the composition and manufacturing conditions. Therefore, inthe conventional erasers, it has not been possible to achieve the beststates in all the properties including the erasing property,eraser-scrap-collecting property, touch in use, crack resistant property(rupture resistant property), etc., and the specifications aredetermined by taking into account the balance of those properties so asnot to raise any problem in practical use.

Here, Japanese Unexamined Patent Publication No. 8-258493 proposes aneraser material composed of an eraser base material including avinylchloride resin, a plasticizer and a filler, and aself-abrasion-type porous material. In accordance with this invention,it is possible to improve the strength without impairing the erasingproperty, and to provide an eraser with high toughness.

However, Japanese Unexamined Patent Publication No. 8-258493 definesthat the self-abrasion-type porous material is a composite materialconsisting of layers made of binder layers that are continuous layershaving a structure like a pencil core material and discontinuous layerscomposed of an inorganic constituent material, such as boron nitride,talc and mica, that does not contribute to adhesion to the interfaces.Therefore, this invention has a mechanism in which the inorganicconstituent material, such as boron nitride, talc, and mica, combined bythe binder is allowed to finely crumble like a pencil core due tofriction, etc. against paper, and erases handwritings on the paper incombination with the eraser base material.

Therefore, since the eraser of Japanese Unexamined Patent PublicationNo. 8-258493 uses the porous material containing the inorganicconstituent material, it has an extreme reduction in the elasticity ascompared with conventional erasers, although it has improved strength ascompared with conventional erasers; thus, the reduced elasticity impairsthe erasing property that inherently should be exerted by the erasingbase material. Moreover, the extreme reduction in the elasticity causesan unusual touch as compared with conventional erasers, failing toprovide a good touch in use. Moreover, it is necessary for the erasernot only to be less susceptible to cracking at the time of erasinghandwritings, but also to generate continuous eraser scraps in acollected manner after erasing; however, since the inorganic materialdifferent from the rubber base material is used as the porous material,it is difficult to generate sufficiently collected eraser scraps.

SUMMARY OF THE INVENTION

As a result of intensive studies for achieving the aforementionedobjects, the inventors have found that when an elastic material of aneraser composition containing at least either a rubber component or aresin component is reinforced by a skeleton structure from whichskeleton portions on the surface of an eraser are separated when rubbed,it is possible to obtain an eraser which is superior in thescrap-collecting property and erasing property, has a good touch in use,and is less susceptible to cracking with high toughness. The presentinvention relates to an eraser which is provided with an elasticmaterial of an eraser composition containing at least either a rubbercomponent or a resin component, and a skeleton structure containing theelastic material, from which skeleton portions on the abrasion surfaceof the elastic material are disconnected and separated together with theabrasion of the elastic material when rubbed.

In one preferred embodiment of the present invention, the skeletonstructure is constituted by a porous structural material, in particular,such as a porous structural material of an organic polymer, that isbroken when rubbed, and this skeleton structure and an elastic materialmade of a conventionally known eraser composition are combined to forman eraser; thus, it is possible to obtain an eraser which is superior inthe scrap-collecting property and erasing property, has a good touch inuse, and is less susceptible to cracking with high toughness.

In another preferred embodiment of the present invention, the eraser iscomposed of a porous structural material that is broken when rubbed. Instill another preferred embodiment of the present invention, the eraserhas a composition in which the porous structural material of theskeleton structure is composed of an organic polymer.

In other preferred embodiment of the present invention, the saidskeleton has a continuous structure.

Therefore, the eraser in the preferred embodiment of the presentinvention is allowed to receive a load applied at the time of erasingwith its entire composite body, i.e, between its elastic material of aneraser composition and the skeleton structure that regulates an extremeelastic deformation of the elastic material, that is, in particular, aporous structural material such as an organic polymer. Therefore, theeraser of the present invention has a high strength, and is superior inthe rupture resistant property with high toughness. For this reason, theeraser is less susceptible to cracking even in the case of repeated useand use with a high load. Thus, in the eraser of the present invention,even if the elastic material of the eraser composition is made softerwith less hardness than the elastic material of a conventional erasercomposition, the strength of the eraser as a whole still is improved,the superior elasticity thereof still is maintained higher, and therupture resistant property thereof is still superior, and the eraser isconsequently less susceptible to cracking during use. In other words, inthe present invention, the eraser is allowed to have a high hardness anda low sticking strength so that it becomes superior in the strength andelasticity.

Moreover, the above-mentioned skeleton structure, that is, especially,the porous structural material, in particular, the porous structuralmaterial of an organic polymer, regulates the excessive elasticdeformation in the elastic material of the eraser composition so thatthe eraser as a whole is provided with high elasticity; however, it doesnot give adverse effects on the viscoelasticity that the erasercomposition inherently possesses, thereby allowing the elastic materialof the eraser composition to exert its inherent viscoelasticity.Therefore, even if the elastic material of the eraser composition ismade softer with less hardness than the elastic material of aconventional eraser composition, the strength of the eraser as a wholestill is improved, the superior elasticity thereof still is maintainedhigher, and it is possible to improve the scrap-collecting propertybecause of the softness of the elastic material that is the erasercomposition. In other words, the eraser scraps generated by the eraserof the present invention are continuous, and formed in a collectedmanner; this makes it possible easily to dispose of the eraser scraps,and is preferable from the viewpoint of deposition of eraser scraps.

Moreover, as compared with conventional erasers, the eraser of thepresent invention can be used with a very light touch at the time oferasing, and is superior in the touch in use. This is probably because,when the skeleton structure, that is, in particular, the porousstructural material of an organic polymer, is exposed to the surface ofthe eraser together with the elastic material of the eraser composition,the skeleton structure, that is, in particular, the porous structuralmaterial of an organic polymer reduces friction against the surface ofpaper, while the elastic material of the eraser composition is allowedto exert its inherent erasing property, so that it is possible to reducea load imposed on the user of the eraser. Moreover, since the eraser hasa low sticking strength, it requires less force to cause abrasion (thatis, a force required at the time of erasing), thereby providing a lighttouch at the time of erasing and the subsequent good touch in use.Furthermore, since the porous structural material is used as theskeleton structure, the superior strength and elasticity prevents theeraser from being bent unnecessarily during use, thereby providing asmooth erasing process with a light touch. Therefore, the arrangement inwhich the elastic material of the eraser composition is made softer withless hardness as compared with the elastic material of the conventionaleraser composition makes it possible to provide a smooth, light tough atthe time of erasing.

Moreover, since the elastic material of the eraser composition is madesofter than the elastic material of the conventional eraser composition,it is possible to provide a superior erasing property. Here, theskeleton structure, that is, in particular, the porous structuralmaterial of an organic polymer, as it is, is allowed to exert an erasingperformance, thereby making it possible further to improve the erasingproperty.

In the case when the elastic material of the eraser composition is madesofter with less hardness as compared with the conventional eraser, theabrasion of the elastic material of the eraser composition isaccelerated as compared with the conventional eraser, with the resultthat it is possible to reduce the possibility in which carbon from apencil, etc. adhering to paper is stuck to the eraser main body anddarkens the eraser.

In the case of the above-mentioned skeleton structure, that is, inparticular, the porous structural material of an organic polymer, at thetime of a rubbing process with the eraser, a deformation applied ontothe elastic material of the eraser composition, as it is, is exerted asa force for separating the skeleton portions on the eraser surface ofthe skeleton structure, that is, a force for breaking the skeletonportions, particularly, in the case of the porous structural material.Consequently, the eraser of the present invention exerts its erasingproperty with the elastic material of the eraser composition being worn,while the skeleton portions being separated or broken, and the eraserscraps of the eraser composition are separated and collected whiletaking fragments of the broken and isolated skeleton portions. For thisreason, the eraser scraps, which include the fragments of the skeletonportions of the skeleton structure and the elastic material of theeraser composition that have been collected, are generated, and noeraser scraps consisting of only the skeleton portions are generated.Moreover, in the eraser of the present invention, during the erasingprocess, the abrasion face of the elastic material and the separatingface of the skeleton structure (the breaking face of the porousstructural material) are made coincident with each other, or madevirtually coincident with each other; thus, after the skeleton portionsof the porous structural material have been separated or broken, andcome off, any separated pieces (broken pieces) of the skeleton portionsof the porous structural material remaining in the eraser main bodyhardly are allowed to rise from the surface of the eraser or to formholes after having been pulled out.

As compared with the conventional eraser, the eraser of the presentinvention generates eraser scraps in a collected manner without beingscattered; thus, it has excellent functions and effects in which theeraser is superior in the scrap-collecting property, provides a smootherasing process, and has a good touch in use. Moreover, since the eraseritself has a proper strength, it is less susceptible to damages, and hasa good rupture resistant property. It also has a good erasing property.

In the preferred embodiment in which the porous structural material isused as the skeleton structure, the structural material preferablycontains the cross sectional shape with virtually polygonal or virtuallycircular cells, and in particular, the porous structural material ismost preferably provided as a foamed structural material.

Moreover, in a still another preferred embodiment of the presentinvention, the porous structural material is a mesh structural material,and most preferably, the porous structural material is athree-dimensional mesh structural material.

The objective of the present invention is to provide an eraser which hasa sufficient strength, is less susceptible to cracking with hightoughness, is superior in its scrap-collecting property, has anexcellent erasing property, and can be used with a good touch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron-micrograph showing the surface of an eraser of oneembodiment of the present invention that is enlarged 80 times.

FIG. 2 is a schematic drawing in which the micrograph of FIG. 1 isre-drawn for convenience of explanation.

FIG. 3 is an electron-micrograph showing the surface of a skeletonstructure constituting the eraser of FIG. 1 that is enlarged 200 times.

FIG. 4 is a schematic drawing in which the micrograph of FIG. 3 isre-drawn for convenience of explanation.

FIG. 5 is an electron-micrograph showing the surface of an eraser scrapof the eraser of FIG. 1 that is enlarged 200 times.

FIG. 6 is an electron-micrograph showing the surface of an eraser ofanother embodiment of the present invention that is enlarged 60 times.

FIG. 7 is a graph that shows the relationship among the curingtemperature, the surface hardness and the sticking strength.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to The Figures, the following description will discussembodiments of the present invention. As described above, FIG. 1 is anelectron-micrograph showing the surface of an eraser of one embodimentof the present invention that is enlarged 80 times. FIG. 2 is aschematic drawing in which the micrograph of FIG. 1 is re-drawn withlead lines and reference numbers for convenience of explanation. FIG. 3is an electron-micrograph showing the surface of a skeleton structureconstituting the eraser of FIG. 1 that is enlarged 200 times. FIG. 4 isa schematic drawing in which the micrograph of FIG. 3 is re-drawn withlead lines and reference numbers for convenience of explanation. FIG. 5is an electron-micrograph showing the surface of an eraser scrap of theeraser of FIG. 1 that is enlarged 200 times. FIG. 6 is anelectron-micrograph showing the surface of an eraser of anotherembodiment of the present invention that is enlarged 60 times. Here, theelectron-micrographs of the present invention were taken by using“ERA-8000” (made by ELIONIX INC.).

As illustrated in FIG. 1 and FIG. 2, an eraser 1 is composed of a porousstructural material that is a skeleton structure 2. Here, this skeletonstructure 2 is composed of a porous structural material of an organicpolymer but can be composed of other substances than organic polymer.Further, the skeleton structure 2 contains an elastic material 3 of aneraser composition containing a rubber component or a resin component.Here, in one embodiment of the skeleton structure of the presentembodiment, a skeleton portion 2 a, which has a greater stiffness ascompared with the elastic material 3 of the eraser composition, iscomposed of a foamed structural material serving as a structuralmaterial having virtually polygonal or virtually circular cells whenviewed in cross section. Reference number 2 b represents a void portionof the skeleton structure surrounded by the skeleton portion 2 a.

As described above, in the eraser of the present embodiment, the voidportion 2 b of the skeleton structure 2 contains the elastic material 3of the eraser composition containing the rubber component or the resincomponent, as illustrated in FIG. 3 and FIG. 4. Therefore, since theelastic member 3 of the eraser composition contains the rubber componentor the resin component, and since the skeleton structure 2 is the porousstructural material of an organic polymer, the void portion 2 b of theskeleton structure 2 is filled with the elastic material 3 of the erasercomposition containing the rubber component or the resin component.

The skeleton structure 2 serves as the skeleton structure so as toreinforce the elastic material 3, and allows its skeleton portion 2 a tobe broken when rubbed. In other words, with respect to the skeletonstructure 2, the porous structural material, which allows the skeletonportion 2 a to be broken when rubbed while reinforcing the elasticmaterial 3, is adopted.

With respect to the skeleton structure 2 of the porous structuralmaterial, although not particularly limited, the average thickness ofthe skeleton portion 2 a can be set to, for example, 1 to 100 μm (andmore preferably, 10 to 50 μm). Moreover, although not particularlylimited, the average pore diameter of the void portion 2 b of theskeleton structure 2 can be set to, for example, 10 μm to 3 mm (and morepreferably, 20 μm to 1 mm).

Moreover, with respect to the skeleton structure 2 of the porousstructural material, although not particularly limited, the void rate(pore rate) is set to, for example, not less than 60%, preferably, notless than 80%, and more preferably, not less than 90% (for example, 90to 99.8%). In particular, when the pore rate is set to not less than90%, the thickness of the skeleton portion becomes smaller, making theporous structural material susceptible to breaking when rubbed, as wellas making the abrasion face of the elastic material and the separatingface of the skeleton structure coincident with each other, or virtuallycoincident with each other.

Moreover, in the skeleton structure of the porous structural material,the filling rate of the elastic material of the eraser composition withrespect to the entire volume of the void portions in the porousstructural material is not particularly limited. However, the fillingrate is preferably set in the range from not less than 50% to less than100%. This is because, when the filling rate is less than 50%, there isdegradation in the eraser-scrap-collecting property, the erasingproperty, and the touch in use. When the eraser has the filling rate of100%, the eraser still has a sufficient strength, is less susceptible tocracking with high toughness, is superior in its scrap-collectingproperty, has an excellent erasing property, and can be used with a goodtouch, as compared with the conventional eraser; however, by setting thefilling rate to less than 100%, more preferably 80 to 60%, and mostpreferably, 70%, it is possible to increase the abrasion rate of thesurface of the eraser at the time of erasing, and consequently toincrease the erasing rate.

Here, with respect to erasers in which the filling rates of the elasticmaterial of the eraser composition with respect to the skeletonstructure were set respectively to 100% and 70% (in which the otherconditions were the same), the erasing rate (%) and the abrasion rate(%) were respectively measured. The results showed that in the case ofthe filling rate of 100%, the erasing rate (%) and the abrasion rate (%)of the eraser were 97.6% and 11.4% respectively, and that in the case ofthe filling rate of 70%, the respective percentages were 98.8% and 16.3%that showed improved erasing rate and abrasion rate. Here, the erasingrate and the abrasion rate were measured under the following conditions:The erasing rate was measured in accordance with JIS S 6050. Theabrasion rate was measured by the following method. First, samples wereprocessed into a column shape having a diameter of 12±1 mm (11 to 13 mm)and a thickness of 10±1 mm (9 to 11 mm), and the weight of this wasmeasured. Then, this was attached to an erasing tester described in JISS6050 (however, in the erasing rate of JIS S6050, since the thickness is5 mm, the holder section was modified so as to accept the sample of 12mm). Against test paper as described in JIS S6050, this was rubbed withreciprocation 50 times with a load of 500 g. Next, eraser scrapsgenerated by the rubbing process were removed, and the weight of thesample was measured. The amount of reduction due to the rubbing processwas calculated as a percentage with respect to the weight before therubbing process.

Moreover, the porous structural material of the skeleton structure ispreferably set to have a tensile strength of not more than 3 kgf/cm²,and more preferably, not more than 2 kgf/cm². The eraser having a porousstructural material having a tensile strength exceeding 3 kgf/cm² as itsskeleton structure makes its skeleton portion difficult to be separatedor broken together with the abrasion of the elastic material of theeraser composition at the time of erasing. The value of the tensilestrength was measured in accordance with JIS K 6402. Here, the thicknessof the sample was 10 mm, the dumbbell had #2 shape, and the tensilespeed was 300 mm/min.

Moreover, the porous structural material is preferably set to have anextension of not more than 500%, and more preferably, not more than100%. The porous structural material having an extension exceeding 500%fails to make the abrasion face of the elastic material and theseparating face of the skeleton structure coincident with each otherduring the erasing process, allowing the porous structural material ofthe skeleton structure to rise from the abrasion face, and givingadverse effects on the external appearance. The value of the extensionalso was measured in accordance with JIS K 6402. Here, the thickness ofthe sample was 10 mm, the dumbbell had #2 shape, and the tensile speedwas 300 mm/min.

Moreover, the porous structural material preferably is set to have acompression repulsive force of not less than 0.2 kgf, and morepreferably, not less than 0.7 kgf. The compression repulsive force ofless than 0.2 kgf causes an insufficient stiffness in the eraser as awhole, resulting in difficulty in providing high elasticity. Thecompression repulsive force was provided as a value that was measured bypressing a disk having a diameter of 15.2 mm onto a sample having athickness of 10 mm and compressing the sample by 5 mm at a rate of 7mm/min.

From these, the preferable porous structural material in this embodimentis an eraser having a tensile strength not more than 3 kgf/cm², anextension of not more than 500%, and a compression repulsive force ofnot less than 0.2 kgf.

With respect to the organic polymer of the skeleton structure 2 of thepresent embodiment, a melamine-based resin was used. Moreover, theelastic material 3 was formed by an eraser composition containing avinylchloride-based resin and a plasticizer.

The eraser of the present invention, which is formed as described above,has a sufficient strength and elasticity, is superior in the ruptureresistant property, is superior in its scrap-collecting property (seeFIG. 5), has an excellent erasing property, and can be used with a goodtouch, as compared with the conventional eraser.

Here, the eraser of the present invention is not limited by theabove-mentioned embodiment. With respect to the porous structuralmaterial, which is not particularly limited, any material may be used aslong as it has a skeleton structure having a skeleton portion and voidportions. With respect to such a porous structural material, forexample, a structural material having virtually polygonal or virtuallycircular cells as shown in FIGS. 1 through 4, or a mesh structuralmaterial may be used. Here, the skeleton portion preferably is set toform a skeleton structure having a sufficient hardness in a solid state;however, any material may be used as long as it has a skeleton structurethat can reinforce the elastic material. For example, the porousstructural material may be fused by heat, have its pore shape deformedupon dissolving (being compatible with) with the component in the erasercomposition, become thinner in the skeleton, or form a semi-compatiblestate with the eraser composition.

In the present invention, the shape of the skeleton portion and theshape of the void portions are not particularly limited. As describedabove, the skeleton structure may be any structure that serves as theskeleton structure so as to reinforce the elastic material and alsoallows the skeleton portion to be broken when rubbed. In any case, anymaterial that can restrict the deformation of the elastic material ofthe eraser composition contained in the void portions of the porousstructural material two-dimensionally or three-dimensionally, and reducethe deformation to a predetermined limitation may be suitable.

The structural material containing virtually polygonal or virtuallycircular cells is not particularly limited, and any structural materialmay be used as long as it contains cells having a virtually polygonalshape or a virtually circular shape; and for example, a structuralmaterial having virtually circular cells such as a foamed structuralmaterial (sponge shaped structural material), or a structural materialhaving virtually polygonal cells such as a honeycomb structuralmaterial, may be used. Here, the foamed structural material ispreferably used. In the case of the foamed structural material, the porerate of less than 90% allows its pores to have a spherical or virtuallyspherical shape; however, the pore rate of not less than 90% allows itspores to form a shape like a three-dimensional mesh structure.

In the structural material (in particular, foamed structural material)containing virtually polygonal or circular cells, the state of pores isnot particularly limited, and any of communicated pores or independentpores may be used, and communicated pores (open cells) and independentpores (closed cells) may be mixed with each other. However, taking itinto consideration that the porous structural material of the skeletonmaterial is filled with the elastic material of the eraser composition,communicated pores are more preferable. Here, with respect to thecommunicated pores, virtually any communicated pores may be used, and inthe present invention, those having a rate of communicated pores of notless than 90% (that is, the rate of independent pores is less than 10%)with respect to all the pores are considered to be communicated pores.

Moreover, with respect to the mesh structural material, a planar meshstructural material (that is, a two-dimensional mesh structuralmaterial) may be used; however, a stereoscopic mesh structural material(that is, a three-dimensional mesh structural material) more preferablyis used. The stereoscopic mesh structural material makes it possible toimprove the strength and viscoelasticity of the eraser.

Here, in the present invention, the porous structural material of theskeleton structure and the elastic material of the eraser compositionmay be integrated into a composite material. When the skeleton structureand the elastic material are integrated into the composite material, itis possible to provide higher toughness, to increase the elasticity andthus improve the rupture resistant property, and to smooth the surfaceof the eraser after use, thereby making the abrasion face of the elasticmaterial and the separating face of the skeleton structure coincidentwith each other. Of course, it becomes possible to improve the touch inuse, the eraser-scrap-collecting property and the erasing property.

The skeleton structure of the present invention is not particularlylimited; however, it is important for the skeleton structure to containthe elastic material of the eraser composition and to allow its skeletonportion on the abrasion surface of the elastic material to be separatedand broken together with the abrasion of the elastic material of theeraser composition at the time of erasing. From this point of view, theskeleton structure preferably is formed by the porous structuralmaterial. The porous structural material is not particularly limited,any porous structural material may be used as long as it contains theelastic material of the eraser composition and allows its skeletonportion on the abrasion surface of the elastic material to be separatedand broken together with the abrasion of the elastic material of theeraser composition at the time of erasing. For example, a porousstructural material composed by an organic polymer or an inorganicpolymer may be used. With respect to the organic polymer, it can be usedeither alone or in combinations of two or more of them. Examples of theorganic polymers include resins (for example, thermosetting resins,thermoplastic resins, etc.) rubbers, fibers, etc. Of course in the caseof the structural material having a porous structural material withvirtually polygonal or virtually circular cells, resins and rubbers areused, and in the case of the mesh structural material, fibers are used.

Examples of the resin include various resins including thermosettingresins such as melamine resins, epoxy resins, urethane resins, urearesins and phenolic resins, and thermoplastic resins, such asstyrene-based resins including polystyrene, ester-based resins includingpolyester, acrylic resins including polyacrylate, olefin resinsincluding polyethylene and vinylchrolide-based resins includingpolyvinylchloride, and elastomers. Examples of rubbers include naturalrubbers, styrene-butadiene rubber, nitrile-butadiene rubber, etc.Moreover, natural high-molecular porous materials such as sponge mayalso be used. Examples of fibers include natural fibers such as cotton,silk and hemp, and synthetic fibers such as cellulose fibers,ester-based fibers, acrylic fibers, and amide-based fibers.

In particular, in the structural material containing the cross sectionalshape with virtually polygonal or virtually circular cells, when amelamine-based resin is used as the organic polymer, it is possible toform a foamed structural material or a stereoscopic mesh structuralmaterial having a thin thickness of the skeleton portion, a small poresize of void portions and a high pore rate. For this reason, thestructure is easily broken when rubbed, and it is possible to avoidbroken fragments of the skeleton portion from appearing on the surfaceof the eraser main body like whiskers after use and also to make theabrasion face of the elastic material and the separating face of theporous structural material coincident with each other, or virtuallycoincident with each other. Of course, even in the case when a materialother than a melamine-based resin is used as the organic polymer, bycontrolling the thickness of the skeleton portion, the pore diameter ofthe void portions and the pore rate, it is possible to provide astructure that is easily broken when rubbed, and allows the abrasionface of the elastic material and the separating face of the porousstructural material coincident with each other, or virtually coincidentwith each other on the surface of the eraser after use.

The elastic material of the eraser composition is not particularlylimited, and a composition, which is impregnated and absorbed into theporous structural material of the skeleton structure, preferably may beused. More specifically, conventionally known compositions such asplastic-based, rubber-based or elastomer-based compositions, used as theeraser base material, may be adopted.

With respect to the plastic-based eraser composition, for example,various resins, such as thermoplastic resins, thermosetting resins, UVsetting resins, electron-beam setting resins, multi-liquid settingresins (two-liquid setting resins, etc.), catalyst setting resins andfiber element ester, may be used. Among these resins, thermoplasticresins are more preferably used. These resins may be applied in variousstates, such as a dissolved state in a solvent, a dispersed state in asolvent or an emulsified state.

More specifically, vinylchloride-based resins such as polyvinylchloride, vinylchloride-vinylacetate-based resins andvinylchloride-ethylene-vinylacetate-based resins, and vinylacetate-basedresins such as ethylene-vinylacetate resins may be used. In particular,a sol-state composition between the vinylchloride-based resin and aplasticizer is most preferably used as the base material. This isbecause the sol-state composition between the vinylchloride-based resinand a plasticizer has a sufficient fluidity upon being impregnated andabsorbed into the porous structural material of the skeleton structure,and because this is easily cured in the void portions in the porousstructural material of the skeleton structure.

With respect to the plasticizer, any known plasticizer may be used aslong as it can plasticize the contained thermoplastic resin, that is, inparticular, polyvinyl chloride. With respect to the plasticizer,phthalic plasticizers, such as dioctylphthalate and diheptylphthalate,are preferably used.

Besides these, the following additive agents may be used: an abrasivematerial, a filler such as calcium carbonate, magnesium carbonate,silica, talc, clay, diatomaceous earth, quartz powder, alumina, aluminasilicate and mica, metal soap, a barium-zinc-based stabilizer,calcium-zinc-based and magnesium-zinc-based stabilizers, a colorant,perfume, a surfactant, glycols, etc. With respect to the colorant, knownpigments, such as organic pigments, inorganic pigments and fluorescentpigments, and known dyes may be used.

With respect to the plastic-based eraser composition, the rate of theresin (in particular, vinylchloride resin) is not particularly limited;and for example, the rate is set to 10 to 80% by weight, and morepreferably, 20 to 70% by weight, with respect to the entire erasercomposition. The rate of the plasticizer is set to, for example, 10 to80% by weight, and more preferably, 20 to 70% by weight, with respect tothe entire eraser composition. Moreover, the rate of the filler is setto, for example, 0 to 70% by weight, and more preferably, 5 to 40% byweight.

The composition of the rubber-based eraser may include, for example, arubber component, a factice, a softener, sulfur, a vulcanizingaccelerator, a filler, an anti-aging agent, a colorant, and perfume. Thecomposition of the elastomer-based eraser may include, for example, athermoplastic elastomer, a softener, a filler, a stabilizer, a colorantand perfume.

Moreover, in the present invention, a color-changing pigment component(pressure-sensitive color-changing pigment component) contained in apressure-sensitive micro-capsule that is ruptured by frictional force,or a color-changing pigment component (heat-sensitive color-changingpigment component) that changes colors by frictional heat, may be used.

Additionally, in the eraser of the present invention, an arrangement maybe adopted in which at least either of the porous structural materialand the elastic material of the eraser composition is colored. Inparticular, in the case when the porous structural material and theelastic material of the eraser composition are colored in respectivelydifferent colors, not only the eraser main body, but also the eraserscraps have a mixed color so that it is possible to provide aninteresting color appearance, and it is also possible to allow the userto observe and recognize the fact that the eraser is formed such thatthe skeleton structure is taken into the eraser scraps as desired,through its physical properties.

Furthermore, the skeleton structure may be constituted by a plurality ofblocks of porous structural materials. For example, these block-shapedporous structural materials have at least any of the shapes ofspherical, rectangular and plate shapes. In particular, in the case whenan eraser is formed by using a plurality of blocks of porous structuralmaterials as the skeleton structure, since each block is independent, ajoint between the blocks serves as a bending portion at which the erasercan be bent along this joint, if necessary. Conventionally, upon usingan eraser, in order to get a better erasing process, the user sometimeshas cut the eraser with a knife, etc. to produce new corners; however,the present invention eliminates this task, and provides convenientusage in which a joint between the above-mentioned blocks can be bentand cut simply with the hands so as to produce new corners, or toprovide, for example, a small piece of the eraser, if necessary. Here,in the case of the eraser having the block-type skeleton structure, inorder to provide easy bending at a joint between the blocks, thecomponents in the eraser composition may be adjusted, the degree ofpolymerization of the synthetic resin may be adjusted, or the gellationtemperature may be adjusted. These methods may be adopted depending onthe type of the main component of the eraser, such as theelastomer-based, vinylchloride-based, or other types of resins. Forexample, in the case of the elastomer-based eraser, an oil component maybe increased as one component of the eraser composition, or the degreeof polymerization of the synthetic resin may be adjusted. Moreover, forexample, in the case of the vinylchloride-based eraser composition, thegellation temperature of the eraser composition contained in theskeleton structure preferably may be set to a comparatively lowtemperature range, such as, 100 to 110° C., more preferably, 105 to 108°C., and most preferably, approximately 107° C. Here, the above-mentionedblocks are not particularly limited; however, it is preferable to setthe diameter or the length of the side thereof to not less than 5 mm.Moreover, the shapes of the blocks may be selected from various shapes,in addition to the spherical, rectangular and plate shapes.

The erasers of the present invention, described in the variousembodiments, effectively may be applied to, for example, a mechanicalpencil with an eraser attached to the end thereof and an electric-typeeraser with an eraser attached to an eraser holder, as well as to afeeding-type eraser having a feeding mechanism, a knocking type eraser,that is, the lead of a mechanical pencil is the eraser itself and aneraser detachably attached into a cylinder. In these cases, the eraserof the present invention is applied, particularly, as an exchange eraser(exchange rubber eraser).

The manufacturing method of the eraser of the present invention is notparticularly limited. For example, a pre-cure eraser composition and acomponent or a structural material to form a skeleton structure aremixed with each other so that the pre-cure elastic material of theeraser composition is impregnated into the void portions of the porousstructural material, and this then is cured so as to provide an erasercomposition. Here, a process in which the material is put into apredetermined eraser mold may be carried out at any time in thepreparation processes.

A preferable manufacturing method of an eraser is explained as follows:an eraser composition containing at least either a rubber component or aresin component is impregnated into a skeleton structure so that theeraser composition is absorbed in void portions in the skeletonstructure, and the eraser composition then is cured. Moreover, anothermethod is explained as follows: an eraser composition containing atleast either a pre-cure rubber component or resin component isimpregnated into a porous structural material so that the erasercomposition is absorbed in void portions in the porous structuralmaterial, and the eraser composition then is cured. Here, in thismethod, after the eraser composition has been impregnated into theskeleton structure, that is, in particular, the porous structuralmaterial, a compressing process is preferably provided. In particular, amost preferable manufacturing method for an eraser includes thefollowing steps: An eraser composition containing at least either arubber component or a resin component is filled into a plate-shapedmolding frame (for example, a plate-shaped molding frame with a bottom),and a porous structural material is placed into the molding frame so asto be impregnated, and this is then compressed with a heating press toprovide an eraser. Here, in order to make uniform and increase theamount of impregnation of the eraser composition, the eraser compositionis further impregnated into the porous structural material into whichthe eraser composition has been impregnated by having been placedthereon; this method is also effective. Moreover, another preferablemethod includes the following steps: The eraser composition containingat least either a rubber component or a resin component is added to theskeleton structure under normal pressures, and the void portions of theskeleton structure as vacuums are allowed to absorb the erasercomposition. Furthermore, in another applicable method, to the skeletonstructure placed under a vacuum is added an eraser compositioncontaining at least either a rubber component or a resin component sothat the void portions of the skeleton structure are filled with theeraser composition, and the eraser composition then is cured.

Additionally, an eraser composition is impregnated into a porousstructural material of the skeleton structure having a size greater thana finished molded size, and this is compressed into a predeterminedmolded product; thus, this method is more preferable in that the porerate and the content of the eraser composition can be adjusted even inthe case when the same porous structural material is used, and in thatit is possible to control the quality of the product properly.

In another applicable manufacturing method, an eraser compositionpreliminarily is impregnated into the void portions of a porousstructural material, and this then is put into a predetermined erasermold where it is cured by applying heat, etc. to provide the product.

With respect to the curing conditions by heat in the above-mentionedvarious manufacturing methods, they are preferably set in a temperaturerange of 100° C. to 160° C. for 10 to 50 minutes.

Here, in the case when the eraser composition is impregnated into theporous structural material of the skeleton structure, if a thermoplasticsynthetic resin such as an elastomer-based resin is contained in thecomposition, the eraser composition sometimes comes to have a highviscosity even in a heated molten state; and in such a case, in order toimprove the impregnating property, the following methods may be used:For example, in this method, after the eraser composition has beendissolved in a solvent and impregnated into the skeleton structure ofthe porous structural material, the solvent is evaporated. Moreover,after the eraser composition has been emulsified by a dispersant, andimpregnated thereto, the dispersant may be eliminated by evaporation,etc. Furthermore, after the eraser composition has been impregnated intothe skeleton structure of the porous structural material in a liquidlow-molecular state, the monomer of the eraser composition may bepolymerized.

Moreover, in the case of the mesh structural material, the same methodas the method for processing the porous structural material containingvirtually polygonal or virtually circular cells may be used, or anothermethod may be used in which: a pre-cure eraser composition in a solstate, etc. is injected into a predetermined eraser mold, and to this isadded fiber and mixed so that the elastic material of the erasercomposition is contained in void portions between the fiber, and this isthen cured by heat, etc. to provide the product.

Upon manufacturing the eraser of the present invention by using theabove-mentioned methods, for example, an eraser composition in a solstate, which has a viscosity in the range of 100 to 20,000 mPa s (morepreferably, 800 to 7,000 mPa s) under the measuring conditions of, forexample, a temperature of 20° C., the application of a B-type viscometerand the rotation speed of 6 rpm, is preferably used, and in particular,a sol-state eraser composition made from a polyvinylchloride resin ismore preferably used. This is because the eraser composition having aviscosity in this range exerts a preferable fluidity in normaltemperatures, so as to be impregnated and absorbed into the voidportions of the porous structural material of the skeleton structure,and properly is filled into the void portions of the porous structuralmaterial of the skeleton structure and easily cured. Additionally, evenan eraser composition having a high viscosity exceeding 20,000 mPa s maybe used and impregnated, with a reduced viscosity by heat or under areduced pressure.

The surface hardness of the eraser of the present invention is notparticularly limited; and, for example, it is set in the range of 50 to80, and more preferably, 60 to 70. Further, the sticking strength of theeraser is not particularly limited; and, for example, it is set in therange of 1.5 to 20 (kgf), and more preferably, 2 to 5 (kgf). Here, thesurface hardness is measured in conformity to JIS S 6050. Moreover, thesticking strength is measured as follows: A sample is machined into adisc shape having a thickness of 5 mm and a diameter of 10 mm, and a rodhaving a diameter of 4.4 mm is pressed onto the center of the disc at aspeed of 7 mm/min., and a load at the time when the portion pressed bythe rod has been broken is measured as the sticking strength.

In the case of the eraser of the present invention, even when thesurface hardness is the same, the sticking strength can be reduced ascompared with the conventional eraser. For this reason, it is superiorin the strength and viscoelasticity, and even if the elastic material ofthe eraser composition is soft, the eraser is less susceptible tocracking with toughness, and has a good touch in use. Of course, it hasa superior eraser-scrap-collecting property, and eraser scraps areproperly collected on the surface of paper or on the surface of theeraser without being scattered. Moreover, it also has a superior erasingproperty.

As compared with conventional erasers having the same erasercomposition, produced through the same manufacturing conditions (curingtemperature, etc.), the eraser of the present invention has lowersticking strength. In other words, the formation of the skeletonstructure makes the elastic material of the eraser composition softereven in the case of the same manufacturing conditions. Thus, althoughthe present invention only adds the porous structural material of theskeleton structure to the conventional eraser composition withoutchanging manufacturing conditions, it becomes possible to provide aneraser that is superior in the strength and elasticity, has a gooderaser-scrap-collecting property, is superior in the touch in use andexerts a high erasing property.

The coefficient of friction of the eraser of the present invention ispreferably set not more than 0.8. This is because the eraser of thepresent invention having a coefficient of friction not more than 0.8 hasa light touch upon erasing. Moreover, the wear rate of the eraser ispreferably set to not less than 1%. This is because the eraser of thepresent invention having a wear rate of not less than 1% is lesssusceptible to stain on its surface upon erasing, and can carry out anerasing process easily.

As described above, it is most preferable for the eraser of the presentinvention to have a surface hardness of 50 to 80, a sticking strength of1.5 to 20 (kgf), a coefficient of friction of not more than 0.8 and awear rate of not less than 1%.

Additionally, the eraser of the present invention is constituted by astructural body having a skeleton structure and an elastic material ofan eraser composition. Therefore, the present invention includes notonly an eraser in which the skeleton structure is contained in theentire elastic material of the eraser composition, but also anothereraser in which the skeleton structure is contained in only one portionthereof. Moreover, as shown in FIG. 6, an eraser 1 in which an elasticmaterial 3 is contained in only one portion of a skeleton structure 2may be included.

EXAMPLES Eraser Material

In the following Examples and Comparative Examples, a sol composition ofpolyvinylchloride having the following composition was used as an erasermaterial (eraser base material).

Composition of Polyvinylchloride Sol

-   -   Resin (polyvinylchloride, trade name “ZEST P21” made by SHIN        DAI-ICHI VINYL CORPORATION)        -   : 32 parts by weight    -   Plasticizer (dioctylphthalate, trade name “SANSOCIZER DOP” made        by New Japan Chemical Co., Ltd.)        -   : 50 parts by weight    -   Filler (calcium carbonate heavy, made by BIHOKU FUNKA KOGYO        Co.,Ltd.)        -   : 17 parts by weight    -   Stabilizer (magnesium-zinc-based material, trade name “R-23L”        made by TOKYO FINE CHEMICAL Co.,Ltd.)        -   : 1 part by weight

Example 1

Foam material (0.15 parts by weight) of a melamine-based resin wasimpregnated with the above-mentioned polyvinylchloride sol composition(20 parts by weight), and this then was heated at 130° C. for 20 minutesto prepare an eraser. Foam material of a melamine-based resin ismanufactured by BASF AG under the tradename of Basotect.

Example 2

Foam material (0.15 parts by weight) of a urethane-based resin wasimpregnated with the above-mentioned polyvinylchloride sol composition(20 parts by weight), and this then was heated at 130° C. for 20 minutesto prepare an eraser.

Foam material of a urethane-based resin is manufactured by INOACCORPORATION under the tradename of MF-50.

Example 3

Foam material (0.15 parts by weight) of an ethylene-based resin wasimpregnated with the above-mentioned polyvinylchloride sol composition(20 parts by weight), and this then was heated at 130° C. for 20 minutesto prepare an eraser. Foam material of an ethylene-based resin ismanufactured by SANWA KAKO Co.,Ltd. under the tradename of OPCELLLC-300#3.

Example 4

Foam material (0.15 parts by weight) of a vinylchloride-based resin wasimpregnated with the above-mentioned polyvinylchloride sol composition(20 parts by weight), and this then was heated at 130° C. for 20 minutesto prepare an eraser.

Example 5

Foam material (0.15 parts by weight) of nitrile-butadiene rubber (NBRfoam) was impregnated with the above-mentioned polyvinylchloride solcomposition (20 parts by weight), and this then was heated at 130° C.for 20 minutes to prepare an eraser.

Example 6

Fiber aggregate in a felt form (0.15 parts by weight) was impregnatedwith the above-mentioned polyvinylchloride sol composition (20 parts byweight), and this then was heated at 130° C. for 20 minutes to preparean eraser.

Fiber aggregate in a felt form is manufactured by TSUKASA FELT SHOJICo.,Ltd. under the tradename of #4000.

Example 7

Foam material (0.15 parts by weight) of a melamine-based resin wasimpregnated with the above-mentioned polyvinylchloride sol composition(20 parts by weight), and this then was heated at 114° C. for 20 minutesto prepare an eraser. Foam material of a melamine-based resin ismanufactured by BASF AG under the tradename of Basotect.

Comparative Example 1

Only the above-mentioned polyvinylchloride sol composition was used, andheated at 130° C. for 20 minutes to prepare an eraser.

(Evaluation)

With respect to the erasers related to Examples 1 to 7 and ComparativeExample 1, evaluation was made on the touch in use anderaser-scrap-collecting property, based upon the following testconditions. The results of the evaluation are shown in Table 1.

TABLE 1 Comparative Examples Example 1 2 3 4 5 6 7 1 Touch in use ⊚ ◯ ◯◯ ◯ ◯ ⊚ × Eraser scrap- ◯ ◯ ◯ ◯ ◯ ◯ ◯ × collecting property(Tests on Touch in Use)

Handwritings written with a pencil were erased by using the erasersrelated to Examples 1 to 7 and Comparative Example, and the touch in usewas evaluated based on the following criteria.

[Evaluation Criteria of Touch in Use]

⊚: Handwritings can be erased very smoothly with a light touch.

◯: Handwritings can be erased smoothly with a light touch.

×: Handwritings cannot be erased without applying a high pressure.

(Tests on Eraser-Scrap-Collecting Property)

After the above-mentioned erasing tests, the eraser-scrap-collectingproperty was evaluated based upon the following criteria. Further, afterthe erasing process, the state of the eraser surface was observed.

[Evaluation Criteria of Eraser-Scrap-Collecting Property]

◯: Eraser scraps are collected on the surface of paper or on the surfaceof the eraser, and after the erasing process, the surface of the eraseris white.

×: Eraser scraps are hardly collected on the surface of paper or on thesurface of the eraser, and after the erasing process, black carbon fromthe pencil adheres to the surface of the eraser.

(Results of Evaluation)

As shown in Table 1, although the erasers of Examples 1 to 7 have hightoughness, they can erase handwritings completely in a manner likesliding with a light touch. Moreover, the eraser scraps are notscattered, and collected on the surface of paper or the surface of theeraser in a continuous form. Moreover, although there is an increase inthe abrasion loss of the elastic material, the pencil handwritings aresufficiently taken into the eraser scraps, and the surface of the eraseris still white. Furthermore, during the erasing process, the skeletonportion of the porous structural material is broken and it is observedthat while eraser scraps are taking in the fragments of the skeletonportion, they are separated in a collected manner.

In contrast, the eraser of Comparative Example 1 cannot erase withoutapplying a high pressure. Further, eraser scraps scatter all over, andare not collected in a continuous form. Moreover, after the erasingprocess, the surface of the eraser turns black with carbon from thepencil adhering thereto. Furthermore, in the case of the erasers ofExamples 1 to 7, even if they are rubbed against the surface of paperwith a high strength, they are not easily broken; however, the eraser ofComparative Example 1 is broken easily when rubbed against the surfaceof paper with a high strength, and therefore, inferior in the ruptureresistant property.

Here, the composition of the elastic material of the eraser compositionand the manufacturing conditions are the same between Examples 1 to 7and Comparative Example 1. Examples 1 to 7 are only different fromComparative Example 1 in that the porous structural material is usedtherein.

Next, a pre-cure eraser of Example 1 using the foam material of amelamine-based resin and a pre-cure eraser of Comparative Example 1which has no skeleton structure of the porous structural material weresubjected to measurements to find the respective relationship among thecuring temperature, the surface hardness and the sticking strength, andthe results are shown in Table 2. FIG. 7 is a graph showing therelationship among the curing temperature, the surface hardness and thesticking strength. In Table 2, the results related to the eraser ofExample 1 are described in the column of “with foam material” and thoserelated to the eraser of Comparative Example 1 are described in thecolumn of “without foam material”. In FIG. 7, represents the stickingstrength of the eraser related to Example 1; ◯ represents the stickingstrength of the eraser related to Comparative Example 1; ▪ representsthe surface hardness of the eraser related to Example 1; and □represents the surface hardness of the eraser related to ComparativeExample 1.

Here, the surface hardness was measured in conformity to JIS S 6050.Moreover, the sticking strength is measured as follows: A sample ismachined into a disc shape having a thickness of 5 mm and a diameter of10 mm, and a rod having a diameter of 4.4 mm is pressed onto the centerof the disc at a speed of 7 mm/min., and a load at the time when theportion pressed by the rod has been broken is measured as the stickingstrength. Here, the curing time was 20 minutes.

TABLE 2 Curing Hardness Sticking strength (kgf) tempera- With foamWithout foam With foam Without foam ture material material materialmaterial 100 42 30 0.50 0.19 110 64 60 1.63 1.73 120 67 66 2.72 3.13 13067 67 3.26 4.15 140 66 65 6.09 7.33

From Table 2 and FIG. 7, within a curing temperature range of 110 to130° C., the eraser of Example 1 has a higher surface hardness and alsohas a lower sticking strength than the eraser of Comparative Example 1.More specifically, the eraser of Example 1 has a surface hardness of 64to 67 and a sticking strength of 1.63 to 3.26 (kgf). This is because,even if it is cured at the same curing temperature for the same time asthe eraser of Comparative Example 1, the eraser of Example 1 has asufficient strength with toughness, and in contrast, also has a lowdegree of curing (degree of gelation), thereby indicating that theelastic material of the eraser composition is softer than that ofComparative Example 1. Therefore, as compared with the eraser of theconventional Comparative Example 1, since the eraser of Example 1 has alower degree of curing and a softer elastic material, it is far superiorin the erasing property, and has a good eraser-scrape-collectingproperty. These trends are also applicable to the erasers ofabove-mentioned Examples 2 to 7. Moreover, in fact, the eraser, whichhas a surface hardness of 64 to 67 and a sticking strength of 1.63 kgfto 3.26 (kgf), was superior in all the factors, such as the strength,touch in use, erasing property, and eraser-scrap-collecting property.

The present invention relates to an eraser which is provided with anelastic material of an eraser composition containing at least either arubber component or a resin component, and a skeleton structurecontaining the elastic material, from which skeleton portions on theabrasion surface of the elastic material are disconnected and separatedtogether with the abrasion of the elastic material when rubbed. Inparticular, in the eraser of the present invention, the skeletonstructure is constituted by a porous structural material that is brokendue to abrasion. Moreover, in the eraser of the present invention, theporous structural material of the skeleton structure is made of anorganic polymer.

Therefore, as compared with the conventional eraser, the eraser of thepresent invention has a sufficient strength so that it is lesssusceptible to cracking even after repeated use, erases with a lighttouch so that it is superior in the touch in use, and also has asuperior erasing property. Moreover, the eraser has a particularlyremarkable effect in that eraser scraps are generated in a continuouscollected form without being scattered.

1. An eraser for erasing information, comprising: a skeleton structureconstituted by a skeleton portion and a void portion; and an eraser-basematerial filled in the void portion, wherein the skeleton portion of theskeleton structure is constituted by a porous structural material oforganic polymer consisting of melamine-based resin, the skeleton portionof the skeleton structure has an average thickness from 1 μm to 100 μm,and the void portion of the skeleton structure has an average pore sizeof 20 μm to 3 mm, the eraser base material is selected from the groupconsisting of (a) a plastic-based eraser composition, (b) a rubber-basederaser composition, and (c) an elastomer-based eraser composition, and(a) the plastic-based eraser composition comprises at least one of avinylchloride resin and a vinylacetate resin, and comprises a filler,(b) the rubber-based eraser composition comprises a rubber component,factice, sulfur, a vulcanizing accelerator, and a filler, (c) theelastomer-based eraser composition comprises a thermoplastic elastomerand a filler.
 2. The eraser of claim 1, wherein a filling rate of theeraser base material is set in the range of less than 100% with respectto the entire volume of the void portion in the porous structuralmaterial.
 3. The eraser of claim 1, wherein a filling rate of the eraserbase material is set in the range from 60% to 80% with respect to theentire volume of the void portion in the porous structural material. 4.The eraser of claim 1, wherein the skeleton portion of the skeletonstructure has an average thickness of 10 μm to 50 μm, the void portionof the skeleton structure has an average pore size of 10 μm to 3 mm. 5.The eraser of claim 1, further comprising a surface hardness of 50 to 80as measured according to JIS S6050, a sticking strength of 1.5 to 20kgf, a coefficient of friction of not more than 0.8 and a wear rate ofnot less than 1%.
 6. The eraser of claim 1, wherein the skeletonstructure is continuous.
 7. The eraser of claim 1, wherein the eraserbase material has a filling rate in a range from not less than 50% toless than 100% with respect to an entire volume of the void portion ofthe porous structural material.
 8. The eraser of claim 1, wherein theporous structural material contains a cross-sectional shape withvirtually polygonal or virtually circular cells.
 9. The eraser of claim1, wherein the porous structural material is a foamed structuralmaterial.
 10. The eraser of claim 1, wherein the porous structuralmaterial is a mesh structural material.
 11. The eraser of claim 1,wherein the porous structural material is a stereoscopic mesh structuralmaterial.
 12. The eraser of claim 1, wherein the porous structuralmaterial has a tensile strength of not more than 3 kgf/cm².
 13. Theeraser of claim 1, wherein the porous structural material has anextension percentage of not more than 500%.
 14. The eraser of claim 1,wherein the porous structural material has a compression repulsive forceof not less than 0.2 kgf.
 15. The eraser of claim 1, wherein the porousstructural material has a tensile strength of not more than 3 kgf/cm²,an extension percentage of not less than 500%, and a compressionrepulsive force of not less than 0.2 kgf.
 16. The eraser of claim 1,wherein at least one of the porous structural material and the eraserbase material is colored.
 17. The eraser of claim 1, wherein theskeleton structure is constituted by a plurality of blocks of porousstructural materials.
 18. The eraser of claim 17, wherein the blockshave at least one shape selected from the group consisting of spherical,polygonal, and plate shapes.
 19. The eraser of claim 1, furthercomprising an exchanging-use eraser of a type selected from the groupconsisting of a feeding-type eraser, a knocking-type eraser, an eraserattached to an end of a mechanical pencil, and an electric-type eraser.20. The eraser of claim 1, wherein the plastic-based eraser compositioncomprises a cured material of a sol-state composition by thevinylchloride resin and a plasticizer.
 21. The eraser of claim 1,wherein the filler is at least one chemical compound selected from thegroup of calcium carbonate, magnesium carbonate, silica, talc, clay,diatomaceous earth, quartz powder, alumina, alumina silicate, and mica.22. The eraser of claim 1, wherein the porous structural material of theskeleton portion is susceptible to breaking when rubbed.
 23. The eraserof claim 1, further comprising a void rate of the skeleton structure isnot less than 90%.
 24. An electric-eraser comprising: an eraser holder;and an eraser for erasing information, the eraser being attached to theholder, the eraser comprising: a skeleton structure constituted by askeleton portion and a void portion; and an eraser-base material filledin the void portion, wherein the skeleton portion of the skeletonstructure is constituted by a porous structural material of organicpolymer consisting of melamine-based resin, the skeleton portion of theskeleton structure has an average thickness from 1 μm to 100 μm, and thevoid portion of the skeleton structure has an average pore size of 20 μmto 3 mm, the eraser base material is selected from the group consistingof (a) a plastic-based eraser composition, (b) a rubber-based erasercomposition, and (c) an elastomer-based eraser composition, and (a) theplastic-based eraser composition comprises at least one of avinylchloride resin and a vinylacetate resin, and comprises a filler,(b) the rubber-based eraser composition comprises a rubber component,factice, sulfur, a vulcanizing accelerator, and a filler, (c) theelastomer-based eraser composition comprises a thermoplastic elastomerand a filler.
 25. The electric-eraser of claim 24, wherein the filler isat least one chemical compound selected from the group of calciumcarbonate, magnesium carbonate, silica, talc, clay, diatomaceous earth,quartz powder, alumina, alumina silicate, and mica.
 26. Theelectric-eraser of claim 24, wherein at least one of the porousstructural material and the elastic material is colored.
 27. Theelectric-eraser of claim 24, wherein the skeleton structure isconstituted by a plurality of blocks of porous structural materials. 28.The electric-eraser of claim 24, wherein the skeleton structure isconstituted by a plurality of blocks of porous structural materials, andthe blocks have at least one shape selected from the group consisting ofspherical, polygonal, and plate shapes.